Lanthanide-based catalyst systems are known to be useful for polymerizing conjugated diene monomers to form polydienes having high cis-1,4-linkage contents, low 1,2-linkages, and linear backbones. These cis-1,4-polydienes containing a linear backbone are believed to provide better tensile properties, higher abrasion resistance, lower hysteresis, and better fatigue resistance as compared to the cis-1,4-polydienes prepared with other catalyst systems such as titanium-, cobalt-, and nickel-based catalyst systems. Therefore, the cis-1,4-polydienes made with lanthanide-based catalysts are particularly suitable for use in tire components such as sidewalls and treads.
However, one disadvantage of the cis-1,4-polydienes prepared with lanthanide-based catalysts is that the polymers exhibit high cold flow due to their linear backbone structure. The high cold flow causes problems during storage and transport of the polymers and also hinders the use of automatic feeding equipment in rubber compound mixing facilities.
Cis-1,4-polydienes synthesized with lanthanide-based catalyst systems may also display pseudo-living characteristics in that, upon completion of the polymerization, some of the polymer chains possess reactive ends that can react with certain functionalizing agents to yield functionalized cis-1,4-polydienes. These functionalizing agents have been employed to improve the cold flow resistance of the resulting polydienes. Nevertheless, whether a particular functional group imparted to a polymer can improve the cold flow resistance or reduce hysteresis is often unpredictable. Furthermore, functionalizing agents that work for one type of polymer do not necessarily work for another type of polymer, and vice versa.
Therefore, there is a need to develop a process for producing functionalized cis-1,4-polydienes having a combination of high cis content and a linear backbone as well as improved cold flow resistance. This combination of properties in a polymer will provide a tire with superior performance.